1. Field of the Invention
Traditionally hearing aid devices have been worn behind the ear, with a sound-conducting tube connected to an earmould placed in the concha and meatus of the hearing-impaired. More recently miniaturization has allowed the electro-acoustic means to be inserted into the earmould, the so-called in-the-ear hearing aids, filling out partly or wholly the concha. Further developments have allowed the size to be diminished so that the entire earmould/hearing aid is substantially retained within the meatus, with only a minor part of the device protruding into the concha, the so-called canal-aids.
A canal-aid has two main advantages: the acoustics are beneficial to the hearing-impaired, due to diffraction around the pinna, and the aid is inconspicuous in use.
2. Description of the Related Art
At present there are the following two techniques available for manufacturing canal-aids:
1. Based on an accurate impression of the individual ear, a shell is cast in acrylics, or produced in metal by electroplating, and the electro-acoustic components of the device are installed in this individually produced shell. PA1 2. The aid is manufactured with a standard case resembling a typical ear canal, and is either inserted directly in the ear canal, or fitted with one of several size adaptor earmoulds which are standard.
The former is referred to as a custom-made canal-aid, and the latter as a modular canal-aid.
The custom-made canal-aid entails the disadvantage that it is manufactured to the individual ear, normally by a specialist manufacturer. Therefore, long time may elapse before the aid is ready, and the customer must place an order without having tried the aid, furthermore, a custom-made hearing aid tends to be expensive to manufacture.
The modular aid, on the other hand, may not secure a comfortable fit in the ear canal, since the earmould is not made to the individual ear. For the same reason, adequate acoustical seal between ear canal and aid is difficult to achieve, frequently resulting in acoustical feed-back due to the short distance between the locus of the amplified sound and the microphone.